As shown, for example, in FIG. 1, a conventional digital amplifier apparatus 10 of this type (hereinafter simply referred to as a “digital amplifier”) is configured with a pulse width modulation section 11, an amplification section 12 and a lowpass filter (hereinafter abbreviated as “LPF”) 13.
Referring to FIG. 1, the pulse width modulation section 11 modulates an input signal from a tuner 15 connected to an antenna 14 by a pulse width modulation scheme.
The amplification section 12 amplifies by switching operation the output signal subjected to pulse width modulation in the pulse width modulation section 11.
The LPF 13 removes the high frequencies produced by the switching operation, and outputs the output signal to a speaker 16 connected to the digital amplifier apparatus 10.
However, with this conventional digital amplifier 10, if the switching frequency is in the receiving frequency band of the tuner 15, the switching frequency produces noise (hereinafter referred to as “switching noise”) and consequently may cause reception malfunctions in the tuner 15.
As a solution to such malfunctions, a digital amplifier 20 provided with a switching frequency control section 21 for controlling the switching frequency variably based on tuning detection signals provided from the tuner 15, has been known conventionally, as shown in FIG. 2 (e.g., see Patent Document 1).    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-363771